Cyber criminals will bank their Windows XP zero-day vulnerabilities until after Microsoft stops patching the aged operating system next April, a security expert argued today.

Jason Fossen, a trainer for SANS since 1998 and an expert on Microsoft security, said it's simply economics at work.

"The average price on the black market for a Windows XP exploit is $50,000 to $150,000, a relatively low price that reflects Microsoft's response," said Fossen. When a new vulnerability -- dubbed a "zero-day" -- is spotted in the wild, Microsoft investigates, pulls together a patch and releases it to XP users.

If the bug is critical and being widely used by hackers, Microsoft will go "out-of-cycle," meaning it will issue a security update outside its usual monthly Patch Tuesday schedule.

But after April 8, 2014, Microsoft has said it will retire Windows XP and stop serving security updates. The only exceptions: Companies and other organizations, such as government agencies, that pay exorbitant fees for custom support, which provides critical security updates for an operating system that's officially been declared dead.

Because Microsoft will stop patching XP, hackers will hold zero-days they uncover between now and April, then sell them to criminals or loose them themselves on unprotected PCs after the deadline.

"When someone discovers a very reliable, remotely executable XP vulnerability, and publishes it today, Microsoft will patch it in a few weeks," said Fossen. "But if they sit on a vulnerability, the price for it could very well double."

Minus any official patching from Microsoft, XP zero-days and their associated exploits could remain effective for months, maybe even years, depending on how well security software detects and quarantines such attacks.

If Fossen's thesis is correct, there should be signs of bug banking, most notably a sharp reduction in the number of publicly-disclosed or used-in-the-wild XP vulnerabilities during the fourth quarter of 2013 and the first quarter of 2014.

"[Hackers] will be motivated to sit on them," Fossen stressed.

There really aren't precedents to back up Fossen's speculation, he acknowledged, because the last time Microsoft pulled the plug on an edition was July 2010, when it retired Windows 2000. But according to metrics firm Net Applications, at the time Windows 2000 powered just four-tenths of one percent of all PCs.

Windows XP will have a much larger share when it's retired next year: Based on XP's current rate of decline, Computerworld has projected that the old OS will still run between 33% and 34% of the world's personal computers at the end of April 2014.

That would be 80 times the share of Windows 2000 when it retired.

But even with Windows 2000's minuscule share when it left support, there were reports that an edition-specific zero-day was created and sold.

"I heard rumors of a new zero-day being found and sold after the support period expired [for Windows 2000]," said HD Moore, creator of the popular Metasploit penetration testing toolkit and the chief security officer of security company Rapid7. "But there were few if any examples that ended up in the public eye."

Moore agreed with Fossen that XP bugs would be more valuable after April 2014, but contended that all Windows vulnerabilities would jump in value.

"Something more common [three years ago] was backporting new security advisories into functional exploits on Windows 2000," said Moore in an email. "Every time a server-side vulnerability was found in Windows XP or 2003 Server, quite a few folks looked at whether this would also work against Windows 2000. My guess is that the retirement of Windows XP will result in all Windows vulnerabilities being of slightly higher value, especially given the difference in exploit mitigations between XP and newer platforms."